Adhesive bonding in structural elements made of hardwood

When the heating period begins or when buildings are repurposed, there is often damage to the laminated wooden beams. As a result, wood planks can come loose decades later and the supporting capacity of the structure may be diminished. This threat is greater in hardwood because it is more stiff and prone to swelling compared to spruce. How can adhesives and procedures be improved so that adhesive bonding of hardwood can be made more reliable for decades to come?

A number of new developments for applications in timber construction rely on the superior mechanical properties of hardwood. In addition, more and more of it is being made available through forestry. Examples are glued laminated beams of beech or beech laminations in surface layers. As beech has a strong tendency to swell and is rather stiff (high modulus of elasticity), residual stresses acting on adhesive joints of beech rise above the ones in soft wood. This makes it difficult to prove its suitability as glue joints according to current codes. On the one hand, this is due to the fact that the available adhesive systems were developed for conifers and not adjusted to the properties of hardwood. On the other hand, the existing codes are developed for soft adhesive bonds in wood, and it is questionable whether they can be transferred to hardwood.

The aim of the research project was to create better adhesive bondings for beech wood in collaboration with Swiss glue manufacturers. This involved questions about production, ideas for reliable detection methods and the basic description of the tension in adhesive joints bearing hydro-mechanical loads. By means of fracture mechanical approaches and numerical simulations, the researchers determined the delamination and long-term behaviour of glued laminated wooden beams and developed a reliable prediction method. One advantage of the fracture mechanical approache is that—in contrast to the currently used codes—they also take into account size effects of the building component. For this purpose, the researchers first had to clarify how fracture-mechanical properties are linked to humidity. In numerical simulations, the researchers established how the glued hardwood elements behave in the long term under seasonal climate changes. These simulations were compared to experiments with glued laminated beech wood under alternating humidity.

Proving that adhesive joints are secure is essential to any commercial use of hardwood products in Switzerland and abroad. In addition, special Swiss adhesive technology for hardwood would cater to a market segment that is growing rapidly both in Switzerland and abroad. A model that accurately describes the changing properties of different wood species and adhesives under changing climate provides the basis for developing hybrid timber configurations as regards the number and thickness of lamellae and other process parameters, such as moisture differences between lamellae. Due to the slow moisture transport in wood, predicting the behaviour of building components over several decades is only possible using the numerical approach.

By conducting fracture-mechanical tests from fibre level through to glued wood beams from beech, the researchers studied the formation of cracks and growth using different adhesives. They were able to show that the type of linkage between fracture modes depends on moisture. They developed a moisture-dependent, non-linear wood and adhesive model that was able to capture all relevant deformations and loads. By implementing the model into a non-linear finite-element environment, users can calculate the time-lapsed development of tension and moisture profiles in building components. This shows that the link between moisture and load plays a crucial role in the sudden breakage in components decades later. The model offers a wide range of application options. Enhancement with cohesive elements enabled the scientists to simulate delamination and predict the damage caused by changing degrees of humidity in laminates of any size. This approach can be used directly by the manufacturers of cross-laminated timber (CLT) and laminated veneer lumber to determine climate conditions to which they are not suited and the resultant rigidity loss in products.

Reliable timber and innovative wood products for structures: adhesive bonding of structural hardwood elements